Waveguide frequency-band/polarization splitter

ABSTRACT

The invention proposes an optimized solution of a frequency/polarization splifter that requires no adjustment after production and can be produced entirely by moulding. The polarized-wave splitter comprises various components, including a polarization splitter  1  coupled to two filters  3  and  5  via transition regions  2  and  4 . The overall dimensions of the various components are such that the transfer characteristics of the splitter are better than the characteristics resulting from the sum of the characteristics of the components constituting the splitter.

The invention relates to a waveguide frequency-band/polarizationsplitter. More particularly, the invention relates to alinear-polarization splatter that includes waveguide filtering functionsin order to split the transmitted waves and the received waves.

Two-way satellite transmissions use different transmit and receivefrequency bands. It is known to use different transmit and receivepolarizations. Moreover, when a frequency band is allocated, in order tomeet high frequency and polarization separation constraints, it is knownto use a waveguide technology. Hitherto, this type of device has notbeen produced on a large scale and each component is relativelyexpensive to produce.

At the present time, a compact high-performance splitter that can bemass produced for a low cost does not exist.

The invention proposes an optimized solution of a polarization/frequencysplitter that requires no adjustment after production and can beproduced entirely by moulding.

The invention is a polarized-wave splitter comprising variouscomponents. At least one common waveguide has a cross section suitablefor letting at least two different polarizations propagate, the commonwaveguide having first and second ends, the first end constituting acommon input/output. A first slot is placed at the second end of thecommon waveguide, the first slot letting waves propagate with a firstpolarization. A second slot is placed on a lateral part of the commonwaveguide, the second slot letting waves propagate with a secondpolarization. A first transition region provides a change in waveguidecross section. A second transition region provides a change in waveguidecross section. A first waveguide filter has a first end connected to thefirst slot via the first transition region, and a second endconstituting a first individual input/output. A second waveguide filterhas a first end connected to the second slot via the second transitionregion, and a second end constituting a second individual input/output.The overall dimensions of the various components are such that thetransfer characteristics of the splitter, within a transmit band andwithin a receive band, measured, on the one hand, between the commoninput/output and the first individual input/output and, on the otherhand, between the common input/output and the second individualinput/output, are better than the characteristics resulting from the sumof the characteristics of the components constituting the splitter,within the said bands.

The invention will be more clearly understood and other features andadvantages will become apparent on reading the description that follows,the description making reference to the appended drawings in which:

FIG. 1 shows the block diagram of the splitter according to theinvention; and

FIGS. 2 to 5 show the four components that constitute the splitteraccording to the invention.

FIG. 1 shows the block diagram of the splitter according to theinvention. The splitter comprises a common port (or common input/output)that is connected to a waveguide antenna component, such as a horn forexample, and two individual ports (or individual inputs/outputs) thatare connected, on the one hand, to a transmit circuit and, on the otherhand, to a receive circuit. The arrows indicated in FIG. 1 merely havethe purpose of indicating the direction of travel of the waves for agiven transmit or receive configuration. The direction of the arrows maybe reversed without any other modification of the splitter, providedthat the transmit and receive circuits (and bands) are reversed. Apolarization splitter 1 connected to the common port splits the wavescoming from the antenna into two groups of waves having two differentpolarizations, in this case two linear and mutually perpendicularpolarizations. A first transition region 2 is connected to thepolarization splitter 1 in order to transmit (or receive) waves with afirst polarization that come from a first end of a first filter 3. Asecond end of the filter 3 constitutes the first individual port. Asecond transition region 4 is connected to the polarization splitter 1in order to receive (or transmit) waves with a second polarization anddeliver them to a first end of a second filter 5. A second end of thesecond filter 5 constitutes the second individual port.

One conventional approach with this type of device consists in choosingand dimensioning the various components individually and to join themtogether using a waveguide portion of constant cross section and havinga length of at least λg/2, where λg is the wavelength specific to thewaveguide, in such a way that the various components do not mutuallyinterfere. The transfer characteristics of the whole assembly are thenslightly inferior to the sum of the characteristics of the componentstaken individually. “Sum” should be understood to mean the combinationof the characteristics, which is not a mathematical sum but rather theresult of a product of matrices. The various components must thereforebe individually of very high performance so that the resulting assemblycorresponds to the desired performance.

According to the invention, the approach of dimensioning the variouscomponents is performed in an overall manner. Firstly, it is necessaryto define what performance levels, in terms of characteristics, aredesired. As an example, it may be desired to produce a splitter thatoperates in transmit mode within a frequency band between 29.5 and 30GHz and, in receive mode, within a frequency band between 19.7 and 20.2GHz. It may be desired to have a reflection coefficient of less than −30dB for each of the ports, a transmission factor of greater than −0.8 dBbetween the common port and the first individual port with the firstpolarization and in the transmit band, a transmission factor of greaterthan −0.8 dB between the common port and the second individual port withthe second polarization and in the receive band, and a transmissionfactor of less than −30 dB between the common port and the secondindividual port with the first polarization and in the transmit band, atransmission factor of less than −30 dB between the common port and thefirst individual port with the second polarization and in the receiveband, and a transmission factor of less than −60 dB between the firstindividual port and the second individual port, whatever thepolarization.

Next, technical choices based on the prior art are made. Thepolarization splitter 1 is, for example, a waveguide of square crosssection having a lateral slot and a slot at one end. As known from theprior art, the use of a slot requires impedance matching, which iscarried out using steps that produce waveguide/waveguide transitions 2and 4. The filters 3 and 5 are, for example, waveguide filters havingpoles, produced using waveguide E-plane stubs.

Optimization starts from the principle that parasitic resonance, ofcapacitive or inductive type, associated with the various components canbe introduced so as to favourably interact with the polarizationsplitter. The optimization then allows a saving of material to be madesince the stubs used for linking become unnecessary.

The starting point of the optimization corresponds to a standarddimensioning operation. The polarizaton splitter 1 is produced as asquare waveguide using slot coupling according to the rules of the artand covering precisely the Tx (transmit) and Rx (receive) bands with thebest possible performance.

FIG. 2 shows a polarization splitter in perspective (FIG. 2 a) and intwo side views at two different angles (FIGS. 2 b and 2 c). For the sakeof legibility of this FIG. 2 and the following figures, only the activewall of the components will be shown. However, FIG. 2 and the otherfigures correspond to the components resulting from the optimization,and a few details will be explained as we go along.

The polarization splitter 1 is a stub of square cross section, withsides C, one end 10 of which constitutes the common port, the other endbeing blanked off and pierced by a first slot 11 of length a_(f1), widthb_(f1) and thickness e_(f1). A second slot 12 is placed on one side ofthe stub at a distance d_(cc) from the blanked-off end of the stub sothat the waveguide terminates in to a short circuit at the centre of theslot for the wavelength of the guided wave. The second slot 12 has alength a_(f2), a width b_(f2) and a thickness e_(f2). The waveguidelength separating the end 10 from the slot is L_(G).

The choice of dimensions of the square waveguide depends on the cutofffrequency in the Rx band—it is necessary that the fundamental mode bepropagative—and on the number of modes of higher order in the Tx band.In addition, it is necessary to have the smallest possible variation inthe wavelength of the guided wave, which makes matching within the bandeasier. The latter condition means taking a waveguide whose dimensionsare approximately 20% larger than the dimensions of the waveguide at thecut-off for the Rx band.

In the present case, a waveguide having a large side of 7.7 mm gives acut-off frequency of 19.5 GHz; a dimension at least 20% larger, but lessthan 10 mm, is chosen since the TE₂₀ mode then has a cut-off frequencyof 30 GHz. Our choice is therefore C=9.6 mm.

The dimensions of the slots are such that: a_(f)>λ_(m)/2,a_(f)/b_(f)>a/b, and b_(f) is very small, λ_(m) being the meanwavelength of the band to be transmitted, a_(f) being the length of theslot, b_(f) being the width of the slot, and a and b representing thelength and width, respectively, of a standard waveguide within thefrequency band in question, such that only the fundamental mode TE₁₀ canpropagate. The equivalent circuit of such a slot at resonance is givenby the parallel LC equivalent circuit. By progressively increasingb_(f), the resonance condition means that a_(f) must increase at thesame time. Thus, from the known equivalent circuit diagram of the slot,C decreases and L increases, thereby producing the quality factor Q ofthe resonant slot (Q is proportional to the square root of C/L) andtherefore an increase in its bandwidth. This increase in bandwidth is tothe detriment of the matching.

The thickness of the slots must in theory be as small as possible so asto have the best coupling, however from the mechanical standpoint itmust be at least the thickness of the waveguide. The thickness of theslots is therefore chosen to be e_(f1)=e_(f2)=0.5 mm. The thickness ofthe slot has an influence on the coupling selectivity; this is becausethe behaviour is no longer solely resonant and a propagative effectstarts to form. This immediately reduces the selectivity. The firstdimensioning operation carried out according to the rules of the artresults in: a_(f1) = 4.77 mm b_(f1) = 1.96 mm a_(f2) = 7.5 mm b_(f2) =0.66 mm L_(G) = λg = 15 mm d_(cc) = λg/4 = 3.75 mm.

Because of the thickness of the slots, a waveguide effect occurs. It isfor this reason that, in order to improve the matching, it is necessaryto use transitions in quarter-wave steps.

These transitions were dimensioned using the well-known quarter-wavematching technique, such as, for example, that indicated in “Waveguidecomponents for antenna feed systems: Theory and CAD” by Borneman.

There is one step for the first transition 2, corresponding to the firstslot 11, and two steps for the second transition 4, corresponding to thesecond slot 12.

The fact of having a single step at the first slot makes it possible,during the following optimization, to merge the first slot 11 with awaveguide cross section of the first transition region 2, thistransition 2 being distributed over the component corresponding to thepolarization splitter 1 and over the component corresponding to thefirst filter 3. An earth plane 13 is added at the end of the first slot11 so as to produce the step with the stub of the first filter that isin contact with it. However, in terms of the initial data, a transitionregion consisting of a first stub 5.5 mm×1.47 mm in cross section and 6mm in length and a stub 6.6 mm×2.29 mm in cross section and 3.83 mm inlength is used.

The second transition consists of three stubs, two of which are shown inFIG. 3, the third stub merging with the stub of the second filter 5.FIG. 3 a shows the component of the second transition 4 in perspectiveand FIGS. 3 b, 3 c and 3 d show this same component in three side views.A first stub 14 comes into contact with the polarization splifter 1. Thefirst stub 14 has a rectangular cross section with a long side of at,and a short side of b_(t1) and a waveguide length of L_(t1). A secondstub 15 follows the first stub 14. The second stub 15 has a rectangularcross section with a long side of a_(t2) and a short side of be and awaveguide length of L_(t2). A third stub 16 is produced on the secondfilter 5, an earth plane 17 providing continuity over the componentshown in FIG. 3. The third stub 16 has a rectangular cross section witha long side of a_(t3) and a short side of b_(t3) and a waveguide lengthof L_(t3): a_(t1) = 7.9 mm b_(t1) = 2.55 mm L_(t1) = 11.9 mm a_(t2) =8.59 mm b_(t2) = 3.14 mm L_(t2) = 7.8 mm a_(t3) = 9.28 mm b_(t3) = 3.72mm L_(t3) = 6.36 mm.

However, the slots contribute to the overall matching, and they musttherefore be modified according to the quarter-wave transitionjuxtaposing it. An overall simulation of the entire system consisting ofthe polarization splitter 1 and the transitions 2 and 4 is carried out.Next, the dimensions of the slots and of the steps are adjusted so as tobring the measured characteristics back into line with the desiredcharacteristics. The simulations and adjustments are repeated until anacceptable result is obtained.

The splitter exhibits good performance, but does not by itself ensuregood rejection between the Tx and Rx bands. The filters are designed toadd an attenuation that allows the desired characteristics to beachieved.

In the illustrative example, waveguide filters having poles made fromstubs are chosen. The filters were synthesized using the methoddescribed in “Waveguide components for antenna feed systems: Theory andCAD” by Borneman.

The second filter 5 is represented in FIG. 4, FIG. 4 a showing aperspective view and FIG. 4 b showing a side view. The second filter 5has two ends 16 and 18, which correspond to waveguides letting the Rxband propagate; as explained above, one of the ends constitutes thethird stub 16 of the second transition 4. To achieve the requiredperformance levels, a three-pole filter produced from first to thirdE-plane stubs 20 to 22, which is placed on a central waveguide 23, ischosen. The central waveguide is coupled to the ends by two irises 24and 25.

Preferably, the filter is produced so as to be symmetrical with respectto the central axis 26 of the filter, in order to make it as twoidentical moulded half-shells. To make it easier to fit the half-shellsof the filter together and to fit the filter into thefrequency/polarization splitter, a filter that is symmetrical withrespect to a mid-plane 27 is produced. Thus, there is no fittingdirection to be respected—the irises 24 and 25 are identical and thefirst and third stubs 20 and 22 are also identical.

The width a_(t3) of the filter remains constant over the entire length.The various components consituting the filter are therefore defined asfollows:

the first and third stubs 20 and 22 have a length L_(tg1) and a heighth_(tg1);

the second stub 21 has a length L_(tg2) and a height h_(tg2);

the central waveguide has a height h_(gc) and the separation between thestubs corresponds to a length L_(s); and

the irises 24 and 25 have a height h_(l) and a length L_(l).

A dimensioning operation according to the prior art is carried out so asto have, for example, the following starting dimensions: L_(tg1) = 0.96mm h_(tg1) = 7.34 mm L_(tg2) = 0.55 mm h_(tg2) = 6.49 mm h_(gc) = 1.45mm L_(s) = 2.95 mm h_(i) = 1.03 mm L_(i) = 0.63 mm.

The first filter 3 is represented in FIG. 5, FIG. 6 a showing aperspective view and FIG. 5 b showing a side view. The first filter 3has two ends 30 and 31 that correspond to waveguides letting the Tx bandpropagate—as explained above, one of the ends constitutes the secondstub of the first transition 2. To achieve the required performancelevels, a two-pole filter is chosen, this being produced by first andsecond E-plane stubs 32 and 33 connected together via a centralwaveguide 34. The first and second stubs 32 and 33 are coupled to theends 30 and 31 via two irises 35 and 36.

Preferably, the filter is made so as to be symmetrical with respect to acentral axis 37 of the filter so as to make it as two identicallymoulded half-shells. To make it easier to fit the half-shells of thefilter together and to fit the filter into the frequency/polarizatlonsplitter assembly, a filter is produced that is symmetrical with respectto a mid-plane 38. Thus, there is no direction of fitting to berespected—the irises 35 and 36 are identical and the first and secondstubs 32 and 33 are also identical.

The width a_(ff) of the filter remains constant over the entire length.The various components consituting the filter are then defined asfollows:

the ends 30 and 31 have a length L_(fe) and a height h_(fe);

the first and second stubs 32 and 33 have a length L_(ft) and a heighth_(ft);

the central waveguide has a height h_(fgc) and the separation betweenthe stubs corresponds to a length L_(fs); and

the irises 24 and 25 have a height h_(fl) and a length L_(fl).

A dimensioning operation according to the prior art is carried out so asto have, for example, the following starting dimensions: a_(ff) = 7.112mm L_(fe) = 5 mm h_(fe) = 3.556 mm L_(ft) = 2.71 mm h_(ft) = 2.13 mmh_(fgc) = 0.97 mm L_(fs) = 14.47 mm h_(fi) = 1.8 mm L_(fi) = 0.52 mm.

The optimization is then carried out by simulating the system consistingof the polarization splitter 1, the first and second transitions 2 and 4and the first and second filters 3 and 5. The slots 11 and 12 are thenredimensioned, by increasing their lengths a_(f1) and a_(f2) in order toincrease the bandwidth, and therefore also increasing their width b_(f1)and b_(f2). For each step, the H-plane discontinuity (inductive effect)and E-plane discontinuity (capacitive effect) are modified so as to havea matched overall LC circuit. The first stubs 20 and 32 (together withtheir symmetrical stubs 22 and 33) of the filters 3 and 5 are modified,so that the LC circuit equivalent to the first stub is matched to thetransition.

The basic idea consists in introducing a mismatch into the plane of theslot in order to compensate for the mismatch of this slot, both in Txand in Rx mode. The LC character of the slots will be modified so as toobtain the bandwidth, the position of the band and the level of matchingthat are desired, the other parameters being modified in order tocompensate for the mismatches created by the modification of the slots.Such a dimensioning operation results, in the detailed example, in thefirst slot being enlarged so as to merge with the stub of the firsttransition.

As a result, the following final dimensions are obtained: a_(f1) = 5.32mm b_(f1) = 3.556 mm e_(f1) = 0.5 mm e_(f2) = 0.5 mm a_(f2) = 8.43 mmb_(f2) = 1.65 mm L_(G) = 15 mm d_(cc) = 1.09 mm a_(t1) = 8.5 mm b_(t1) =4.17 mm L_(t1) = 0.96 mm a_(t2) = 8.61 mm b_(t2) = 4.318 mm L_(t2) =2.94 mm a_(t3) = 10.668 mm b_(t3) = 4.318 mm L_(t3) = 5.7 mm h_(tg1) =6.56 mm L_(tg1) = 1.36 mm h_(tg2) = 6.81 mm L_(tg2) = 1.21 mm L_(e) =3.42 mm h_(gc) = 1.48 mm L_(i) = 0.8 mm h_(l) = 1.29 mm a_(ff) = 7.112mm L_(fe) = 2.03 mm h_(fe) = 3.556 mm L_(ft) = 2.7 mm h_(ft) = 1.86 mmh_(fgc) = 1.16 mm L_(fs) = 14.14 mm h_(fi) = 1.8 mm L_(fi) = 0.55 mm.

At the end of the process, a set of components (slots, transitions andfilters) that are dimensioned so as to be used in thefrequency/polarization splitter is obtained. However, these components,taken individually, are not efficient in the desired frequency bands. Aperson skilled in the art may even notice that the specificcharacteristics of each component do not allow a priori the overallcharacteristics of the splitter to be obtained since their sum does nota priori allow the final characteristic of the splitter described to beobtained. However, the parasitic interaction of the various componentsdoes make it possible, by carrying out an overall dimensioning operationon the system, to achieve characteristics of a very high level.

The invention is not limited to the embodiment described. A personskilled in the art may change certain elements, while still followingthe same approach. The type of waveguide filter used may be replacedwith any other type of waveguide filter. The square and rectangularwaveguide cross sections may be replaced with circular and ellipticalwaveguide cross sections.

1. Polarized-wave splitter, which comprises at least the followingcomponents: a common waveguide having a cross section suitable forletting at least two different polarizations propagate, the commonwaveguide having first and second ends, the first end constituting acommon input/output; a first slot placed at the second end of the commonwaveguide, the first slot letting waves propagate with a firstpolarization; a second slot placed on a lateral part of the commonwaveguide, the second slot letting waves propagate with a secondpolarization; a first transition region providing a change in waveguidecross section; a second transition region providing a change inwaveguide cross section; a first waveguide filter having a first endconnected to the first slot via the first transition region, and asecond end constituting a first individual input/output; and a secondwaveguide filter having a first end connected to the second slot via thesecond transition region, and a second end constituting a secondindividual input/output; wherein the overall dimensions of the variouscomponents are such that the transfer characteristics of the splitter,within a transmit band and within a receive band, measured, on the onehand, between the common input/output and the first individualinput/output and, on the other hand, between the common input/output andthe second individual input/output, are better than the characteristicsresulting from the sum of the characteristics of the componentsconstituting the splitter, within the said bands.
 2. Splitter accordingto claim 1, wherein the filters are symmetrical with respect to amid-plane.
 3. Splitter according to claim 1, wherein the componentsconstituting the splitter are produced by moulding.